Water based drilling fluids were the first ever used and continue to be used today. Its lower cost and environment acceptance as compared to oil mud continue to make it the first opinion in drilling operations. The following chart is an overview of the performance issues for respective systems.
General MudShaleSevere LostCategoryLubricityReactivityHole CleaningCirculationWater-basedPoorPoorGoodPoorOil-basedGoodVery GoodPoorVery Poor
Daily drilling fluid operations dictate that the fluid engineer make regular chemical treatments to maintain API mud properties. This proposal addresses those times when the treatment of an entire system may either be deemed to expensive or not necessary. The introduction of pre-engineered pills to the system to achieve a desired goal is a long standing operation. For example: whenever the pipe is pulled out of the well to replace the bit, a “slug” is pumped. A slug is a pre-engineered pill of existing mud mixed with barite to increase the density of the pill above that of the entire system. By pumping this pill into the drill pipe, the hydrostatic column of the fluid will fall and when the pipe connections are broken, the pipe will be dry, thus preventing mud from getting on the rig personnel and equipment. These slugs are generally built on location in the slugging pit. Another example of a pill not built on location are spotting fluids. These are pre-engineered blends which are made at a vendor's shop and stored at the wellsite in tanks which can be easily accessed when stuck pipe may occur. This is discussed in U.S. Pat. No. 4,876,017.
This invention utilizes both scenarios to deliver a pre-engineered blend of elastomer and oil for the purposes previously stated.
Lost Circulation—Severe
In U.S. Pat. No. 6,518,224, Wood teaches how the use of elastomer particles added to a an oil based drilling fluid can effectively manage the whole mud losses encountered while drilling. This technique as taught has proven to be very effective yet in situations of 100% losses alternative remedies have been tried. These include, but not limited to, pills spotted at the lost zone comprised of cross-link polymers, diatomaceous earth, fibers and cement. Also combinations of these products have been tried with some or limited success. This invention teaches how the use of a high concentration of elastomer added to the base fluid in a pre-engineered manner can result in a pill that will “lock-up” downhole by absorbing the available oil and thus stopping the flow of free oil into the thief zone. Furthermore the expansion capability of the elastomer, as taught by Wood will then have time to take effect within the fractures to form a flexible seal that allows drilling to resume. The further advantage is that since the pill can be engineered with a compatible fluid to the existing system, when drilling resumes the pill will wash out of the open hole and not contaminate the active drilling fluid. Most of the pills currently used for this application are mixed in water which is a contaminant to oil based fluids. When drilling resumes with these water-based pills in the well, it must either be disposed of or incorporated into the system creating an additional step with associated cost.
This method can also be applied to water based fluids though the applications are not as common. Dependent on the environmental compliance of the drilling site, the pill and its oil can be incorporated into the system with a resulting improvement in lubricity and bit balling as noted in those sections.
Shale Reactivity/Bit balling
One of the primary rocks encountered when drilling is smectite. This rock is highly reactive in an aqueous environment and takes on a “gummy or sticky” texture. As drilling proceeds, this material begins to cling to the drill bit and the bottom hole assembly (BHA). This “mud ring” can completely cover the teeth of the bit thus greatly slowing drilling and impact the BHA in such a way that when pulling pipe out of the hole that the well swabs creating a well control problem. Traditionally, the industry has dealt with this problem by adding oils and surfactants to wash the bit and BHA. Ground particles such as walnut have also been circulated through the well in an attempt to scour the bit and BHA. These practices have shown some success as demonstrated by torque gauges and drill rates monitored on the drilling rig.
Lubricity
There are two parameters of a drilling fluid that can be altered to improve the lubricity of water and oil based muds. First is the lubricity coefficient of the external phase of the system. Secondly is the introduction of particles that become offsets within the wellbore providing a surface for the drilling assembly to contact rather than the casing or formation.
Oil based mud has an inherent advantage over water based systems due to the lower coefficient of friction in the external phase of the mud which is oil versus water. However, even in the oil based muds solid particulate has proven to reduce torque and drag in highly deviated or narrow hydraulic clearance.
In water based mud, the addition of non-aqueous products has been the primary method for increasing lubricity. A limitation of this method is the oils are emulsified into the water fraction after several circulations and the oil must continue to be added in order to remain external in the system thereby being externally available to lubricate the walls of the well.
Solid lubricants have primarily been polymer and glass drill beads. They are touted to form a ball bearing effect downhole. Concerns and limitations with beads are they get circulated out of the hole which is why recycling equipment must be employed and they can pose a safety hazard on the rig to personnel when they are accidentally spilled in the work area.
Hole Cleaning/Cuttings Removal
One of the major functions of a drilling fluid is the removal of cuttings made by the bit. The application of this invention is for the pill to be used in an oil based system. Water based systems operate with a different chemistry and can provide sufficient hole cleaning with existing rheological modifiers. To generate viscosity in oil-based drilling fluids two methods are employed. First, the amount of water emulsified into the oil will increase viscosity and secondly, organophillic clays and polymers are added to improve the rheology for hole cleaning. Various rheological models are employed within the industry to monitor hole cleaning. Examples of these models include but are not limited to Bingham Plastic and Power Law. According to most models, by increasing the density or plastic viscosity of a fluid, the lifting capacity of that fluid will be improved resulting in superior hole cleaning. For this reason, operations often pump a concentrated pill of barium sulfate (barite) which will increase both the density and plastic viscosity to clear the wellbore of cuttings. This pill will normally cover several hundred feet of hole length therefore volumes will vary dependent on the hole geometry. The limitation of this treatment is the increased density of the pill adds to the total hydrostatic pressure which may exceed the formation fracture gradient. In this scenario the high density pill will cause a fracture with resulting loss circulation. Furthermore, these pills create a buildup of density for the entire system when they return to the surface.